Syphilis
How you can get it: Oral sex, contact with an open sore. How common it is: 40,000 cases a year. The symptoms: A painless ulcer that disappears over time, sometimes accompanied or followed by a rash. Is it serious? It's easily treated, but many people think the disease is gone just because the initial sores have healed. Left untreated, it can kill you.
Herpes Simplex
How you can get it: Kissing, oral sex. How common it is: half a million cases a year. The symptoms: Painful lesions that go away and reappear, usually on your mouth or genitals. Most of the time the oral virus is passed to your partner's mouth and the genital virus to the genitals, but in some cases it is transmitted via oral-genital contact. Is it serious? Not deadly. The virus never goes away, and can be spread even when you don't have sores.
Human Papillomavirus
How you can get it: Hand-genital contact. How common it is: 500,000 to a million cases each year. The symptoms: often there aren't any; there may be warts on the genitals, or pain. Is it serious? It is if it leads to cancer. Some of the 70 types of HPV actually cause cervical cancer (this is one of the most important reasons to get regular exams if you're a woman).
Scabies and Pubic Lice (crabs)
How you can get it: skin-to-skin contact. How common it is: Unknown. The symptoms: Scabies are small, pimple-like bumps caused by tiny mites spread through close contact. They itch a lot, particularly after you exercise and in the evenings. Lice are small insects that are spread the same way, usually on your head but sometimes in your pubic hair (aka crabs). Is it serious? No, but ask anyone who's had either one: the itching is enough to drive you crazy.
HIV
How you can get it: oral sex. How common it is: 120,000 new cases last year. The symptoms: sometimes none, some people come down with what seems like mono or the flu within a few weeks of infection. Is it serious? You should know the answer to this one>HIV causes AIDS, for which there's no known cure.
